net: bgmac: drop struct bcma_mdio we don't need anymore

Adding struct bcma_mdio was a workaround for bcma code not having access
to the struct bgmac used in the core code. Now we don't duplicate this
struct we can just use it internally in bcma code.

This simplifies code & allows access to all bgmac driver details from
all places in bcma code.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>